Roadwork is typically carried out by working machines that carry a working implement and travel along a roadway. In the operation of road milling machines and paving machines, it is critical to control the transverse slope and/or the longitudinal grade of the working implement with respect to the roadway. Consequently, conventional road working machines include grade and slope controls that can be used to insure that the working implement will be disposed and will remain disposed at a desired level both along and across the roadway being worked.
Roadway repair is often accomplished by overlaying the existing pavement (whether of concrete or asphalt composition) with a new layer (often called a leveling course) of concrete, asphalt or other surfacing materials. Without prior surface treatment, however, this method of repair generally results in the application of insufficient quantities of paving material in the rutted, potholed or otherwise damaged areas, because the overlay will be applied at the same rate per unit of roadway width in damaged areas (which have a greater depth across the width) as in the undamaged areas. The resulting reduced density in the overlay of the previously damaged areas will lead to renewed rutting or other wear damage in the new pavement in relatively short order. However, by milling the surface of the damaged pavement to a flat surface, the damaged areas will be removed so that newly added pavement will have a uniform thickness across the entire width of the roadway. In addition, a repaving technique that includes milling a thickness of old pavement and replacing it with an equivalent thickness of new pavement will return the elevation of the roadway to its initial level, whereas the placement of a leveling course atop damaged pavement will tend to raise the surface of the roadway or some portion thereof above its original elevation. This can require the raising of road shoulders, guardrails and manhole covers and the adjustment of overpass clearances, all of which is unnecessary if a proper milling technique is employed. A use of milling prior to repaving can also permit ready establishment of the proper road grade and slope, and thereby avoid drainage and safety problems. Furthermore, milling typically provides a rough surface that readily accepts and bonds with the new asphalt or other pavement overlay. Finally, milling can provide raw material that can be reclaimed for use in the production of new paving materials.
A milling machine is typically a wheeled or track-driven vehicle that is provided with a rotating working drum that includes a plurality of cutting teeth. The drum is mounted in a housing on the frame of the machine and adapted to be lowered into contact with the road surface and rotated about a horizontal axis so as to cut into the surface to a desired depth as the machine is advanced along the roadway. Generally, the milling machine also includes a conveyor system that is designed to carry the milled material that has been cut from the roadway by the rotating drum to a location in front of, to the rear of or beside the machine for deposit into a truck for removal from the milling site. One or more spray bars are typically mounted over the conveyors and inside the drum housing so that water may be sprayed to control the dust and heat that is generated in the milling process. In addition, a vacuum system may be provided to assist in removing dust generated in the milling process. Steerable track or wheel drive assemblies are provided to drive the machine and to steer it along a desired milling path. Power for driving the machine and for operating its systems is typically provided by a diesel engine.
It is common for the track or wheel drive assemblies of a milling machine to be mounted at the bottom of vertical columns, and for the frame which carries the milling drum to be adjustable upwardly and downwardly on the columns to change the depth of cut of the milling drum with respect to the surface on which the track or wheel drive assemblies rest. Linear actuators are provided to move the frame with respect to the columns, and sensors are provided to scan the surface of the roadway to provide accurate information about the location of the milling drum or the frame with respect to the surface to be milled Conventional milling machines include a grade and slope control system that can be used to insure that the milling drum will be disposed at a desired level both along and across the roadway being milled as the milling machine is operated. A level milled surface is critical in obtaining a smooth and level paved surface.
When the milling operation has been completed, paving of the roadway with asphalt material is generally carried out by a paving machine that is supplied with asphalt material by a number of supply trucks and/or a material transfer vehicle. The paving machine is self-propelled and driven by a wheeled or tracked drive system. In a common type of paving machine, an asphalt receiving hopper is located at the front end of the machine to receive asphalt material from a truck or material transfer vehicle, and a hopper conveyor typically comprised of one or more slat conveyors located below the asphalt receiving hopper transfers the asphalt material from the hopper to a transverse distributing auger that is mounted near the rear of the machine. The asphalt material is deposited onto and across the roadway or other surface to be paved by the distributing auger. A floating screed located at the rear end of the machine behind the distributing auger compacts the asphalt material and forms the asphalt mat. The floating screed is attached to the paving machine by a pair of tow arms, and a linear actuator is typically provided at the front of each tow arm to adjust the elevation of the floating screed.
As has been mentioned, it is known to provide systems for adjusting and controlling the attitude of the frame (and thus, the elevation and slope of the milling drum) on the front columns that support the drive assemblies of a milling machine. It is also known to provide similar systems for adjusting and controlling the attitude of the screed of an asphalt paving machine. Two closed-loop control systems are typically provided, one for the left front column of the milling machine or the left front tow arm of the paving machine, and the other for the right front column or right front tow arm. Each system includes a depth measuring sensor which measures the distance from the roadway surface to a reference point on the frame of the working machine, and an operator may cause signals to be transmitted to the linear actuators on each of the front locations to adjust the distance of the milling drum or screed from the roadway surface. Each of these linear actuators is operated by an electrically powered valve, and the minimum electrical current that is required to cause the valves to operate the linear actuators in order to affect the attitude of the frame of a working machine varies considerably. It is necessary to calibrate the system to set this minimum electrical current for each linear actuator in order to insure smooth and efficient operation of the attitude control system. However, it is a time-consuming matter to calibrate the conventional systems for grade and slope control in the conventional manner prior to beginning a milling or paving operation. Furthermore, conventional calibration techniques for attitude control systems require significant operator training.
It would be desirable if an automatic system could be provided to insure that calibration of the grade and slope control systems for a working machine can be accomplished quickly and by minimally trained operators.